The present invention relates to an image reading device for an optical disc in which the setting of an optical disc to a related location is detected by a detecting section, information is read out of the optical disc of which the setting is detected, by means of a reading section, and an RF signal output from the reading section is binarized at a predetermined slice level, which is set by slice level setting means.
To read information that is stored in an optical disc, such as CD or DVD, from the disc, an optical disc 1 is set to a spindle motor 2 which is driven and rotating, as shown in FIG. 4. A light beam emitted from an optical pick-up 3 is irradiated onto the optical disc 1. Whether or not the optical disc 1 is set to the motor is detected depending on the presence or absence of light reflected from the optical disc. Upon detection of the setting of the optical disc 1, the pick-up 3 reads out information from the optical disc 1 and then produces an RF signal. The RF signal is amplified by an RF amplifier 4, and binarized at a predetermined slice level for binarizing. The binarized signal output from the RF amplifier 4 is decoded by a decoder 5, and error and jitter contained therein are detected and processed for error correction and the like.
In this case, the slice level in the RF amplifier 4 is set by means of a slice level setting means, which is attached to or contained in a microcomputer (referred frequently to as MICON) 6 for the servo control.
An example of a binarizing circuit in the RF amplifier 4 is constructed as shown in FIG. 5. An RF signal output from the pick-up 3 is input to a non-inverting input terminal of a comparator 8. An output voltage signal of a slice level setting means 9 is input an inverting input terminal of the comparator 8. The voltage signal output from the slice level setting means 9 is basically a voltage signal appearing at a variable terminal of a variable resistor. A voltage level at the variable terminal corresponds to a slice level at the time of the binarizing.
Due to various factors, e.g., error and deformation, caused in forming disc maters and stampers for the optical discs 1 or molding the discs, an optimum slice level for the binarizing frequently offsets from a mid point of the RF signal, viz., a center of a maximum amplitude of the RF signal, and is different for each disc.
Although the optimum slice level is thus different for each optical disc 1, the slice level set by the slice level setting means 9 is set at a fixed value. As a result, the slice level by the slice level setting means 9 shifts from the mid point of the RF signal for the optical disc 1, and the error and jitter are increased in the signal produced by decoding the binarized signal of the RF signal by the decoder 5. Consequently, information is not read out of the optical disc 1 precisely.
Techniques to change the slice level when the binarizing is carried out are described in the Unexamined Japanese Patent Application Publication Nos. Sho 58-203635 and Hei 10-188291. In the former publication, the slice level is followed up so as to minimize an error rate by the utilization of the feedback control. In the latter publication, a slice level signal is generated following up a readout signal, by using the peak and bottom values of the readout signal. In both the techniques, the follow-up control is constantly performed. Therefore, the control procedure is extremely complicated.
In the Examined Japanese Patent Application Publication No. Hei 2-58708, there is a description that an optimum slice level is set corresponding to a PCM (pulse code modulation) signal. Also in this technique, the follow-up control procedure is very complicated. Accordingly, the technique is different from the present invention which sets the slice level so as to minimize the jitter of the RF signal read out of the optical disc.